1. Field of the Invention
The present invention relates to thermal liquid ejection heads for inkjet printers and liquid ejection apparatuses such as inkjet printers including the liquid ejection heads, and more particularly, to a technique for cooling a liquid ejection head, that is, a technique that can reduce thermal variation of the liquid ejection head per unit time.
2. Description of the Related Art
Thermal liquid ejection heads and piezoelectric liquid ejection heads are well known examples of liquid ejection heads used in liquid ejection apparatuses such as inkjet printers. The former utilizes expansion and contraction of bubbles generated by heat, whereas the latter utilizes the variation in shape and volume of piezoelectric elements. The thermal liquid ejection heads include heating elements on semiconductor substrates. When the heating elements heat up, generated heat vaporizes liquid in reservoirs to create bubbles, thereby ejecting liquid drops from nozzles, which are disposed above the heating elements, onto recording media.
FIG. 17 is a perspective view of a liquid ejection head or head 1 of a known type. Although a nozzle sheet 17 is bonded to a barrier layer 3 in an actual configuration, the nozzle sheet 17 is separated from the barrier layer 3 in FIG. 17 and the nozzle sheet 17 and the barrier layer 3 are inverted for convenience. FIG. 18 shows the structure of a flow path of the head 1 shown in FIG. 17.
Referring to FIGS. 17 and 18, a plurality of heating elements 12 is disposed on a semiconductor substrate 11. The barrier layer 3 and the nozzle sheet 17 are disposed on the semiconductor substrate 11 in this order. A head chip la includes the semiconductor substrate 11, provided with the heating elements 12, and the barrier layer 3 disposed on the semiconductor substrate 11. The head 1 includes the head chips 1a and the nozzle sheet 17 bonded onto the head chip 1a. 
The nozzle sheet 17 includes nozzles 18 disposed right above the respective heating elements 12. The nozzles 18 have openings from which ink drops are ejected. Since the barrier layer 3 is disposed between the heating elements 12 and the nozzles 18, reservoirs 3a are formed in the spaces enclosed by the barrier layer 3, the heating elements 12, and the nozzles 18.
As shown in FIG. 17, the barrier layer 3 has a comb-shape when viewed from above. Therefore, three sides of each heating element 12 are enclosed by the barrier layer 3 but one side thereof is open such that this opening serves as an individual flow path 3d, which is connected to a common flow path 23.
The heating elements 12 are aligned in the vicinity of one side of the semiconductor substrate 11. As shown in FIG. 18, since a dummy chip D is disposed on the left side of the semiconductor substrate 11 (head chip 1a), the common flow path 23 is formed between the left side of the semiconductor substrate 11 (head chip 1a) and the right side of the dummy chip D. The dummy chip D may be composed of any component that can form the common flow path 23 with the semiconductor substrate 11.
As shown in FIG. 18, a channel plate 22 is disposed on the side of the semiconductor substrate 11 opposite from the side on which the heating elements 12 are disposed. The channel plate 22 includes an inlet 22a and a supplying flow path 24 communicating with the inlet 22a. The supplying flow path 24 having a rectangular cross section, in turn, communicates with the common flow path 23.
Ink supplied from the inlet 22a passes through the supplying flow path 24, the common flow path 23, and the individual flow path 3d to enter the reservoir 3a. When the heating element 12 heats up, a bubble is generated in the reservoir 3a on the heating element 12. The generated bubble ejects a drop of ink in the reservoir 3a through the nozzle 18.
In FIGS. 17 and 18, dimensions are not to scale and some parts are enlarged to aid understanding. In actual size, the thickness T of the semiconductor substrate 11 shown in FIG. 19 is about 600 to 650 μm, and the thicknesses of the nozzle sheet 17 and the barrier layer 3 are about 10 to 20 μm, for example.
FIG. 19 shows a state in which a droplet is ejected due to the heat by the heating elements 12 disposed in the head chip 1a shown in FIG. 18. Typically, a distance Yn from the center of the heating element 12 to a first side surface of the head chip 1a that faces the dummy chip D is about 100 to 200 μm, whereas the width of the head chip 1a is about ten times larger than the distance Yn, namely, larger by an order of magnitude. That is, the heating elements 12 are disposed close to the first side surface of the head chip 1a. 
In the structure shown in FIGS. 18 and 19, when the heating elements 12 heat up to high temperatures, the temperatures of the heating elements 12 can be hundreds of degrees Celsius at a moment. This generated heat brings liquid on the heating elements 12 to a boil. At this time, the heat also travels through the semiconductor substrate 11 on which the heating elements 12 are disposed. To minimize this energy loss, a heat-insulation layer composed of a material having a low thermal conductivity such as silicon oxide is disposed between the heating elements 12 and the semiconductor substrate 11.
It is the top surface of the semiconductor substrate 11 that the heat traveling through the semiconductor substrate 11 reaches first. The top surface of the semiconductor substrate 11 is flash with the top surface of the heating elements 12 and is in contact with liquid. Secondly, the heat traveling through the semiconductor substrate 11 reaches the first side surface of the semiconductor substrate 11, that is, the surface forming the common flow path 23 with the dummy chip D.
Now, a mechanism of how a bubble is generated in a thermal liquid ejection head will be described. A heater, e.g., the heating element 12 is in contact with liquid such as ink, and thermal energy from the heater heats up the liquid. When the temperature of the heater exceeds the boiling point of the liquid, the liquid boils. From an academic point of view, “boiling” denotes nucleate boiling. More specifically, the surface of the heater has small scratches or dents in which masses of air, which are called bubble nuclei, exist. Bubbles are generated in these bubble nuclei.
Accordingly, even though the heaters are in contact with liquid, generation of bubbles depends on the condition of the surfaces of the heaters at the same temperature. The number of bubble nuclei determines the number of bubbles generated on the surface of the heater. More bubbles are generated on the surface of the heater with many bubble nuclei than on the surface of the heater with a small number of bubble nuclei. That is, bubbles are readily generated on a rough surface but are hardly any generated on a smooth surface.
The surface of the head chip 1a on which the heating elements 12 are disposed is very precisely finished by a semiconductor process and thus is extremely smooth. By contrast, since the first side surface of the head chip 1a is processed through dicing, that is, cutting using, e.g., a rotary saw, the first side surface of the head chip 1a has irregularities and thus bubble nuclei exist therein. FIG. 20 is an enlarged photomicrograph showing the surface of the head 1 and a surface cut through dicing. Hence, bubbles are readily generated in liquid on the first side surface of the head chip 1a. 
To prevent bubbles from being generated on the first side surface of the head chip 1a, the following methods are proposed. A first method is that the heating elements 12 are aligned well remote from the first side surface of the head chip 1a such that it is difficult for the heat generated by the heating elements 12 to reach the first side surface. In this way, thermal energy reaching the first side surface of the head chip 1a hardly brings liquid to a boil.
A second method is that the first side surface of the head chip 1a is made smooth such that irregularities in which bubble nuclei exist are eliminated. A third method, which is disclosed in Japanese Unexamined Patent Application Publication No. Hei 9-11479, is that an ink inlet or opening is formed through anisotropic etching in the center area of the head chip 1a and a heating element is disposed in the vicinity of the ink inlet.
With the first method, since a wide gap is disposed between the first side surface of the head chip 1a and the aligned heating elements 12, the gap makes the head 1 large, which contradicts high-density packaging of the head chip 1a. The second method requires an additional step of processing the surface of the head chip 1a after the head chip 1a is cut through dicing, resulting in increased cost.
With the third method, anisotropic etching is performed on the head chip 1a and thus the surface on which the ink inlet is formed is extremely smooth. Therefore, bubbles do not develop on this smooth surface of the head chip 1a. Unfortunately, since the ink inlet is provided in the center area of the head chip 1a, the head chip 1a has a complex structure. Thus, provision of the ink inlet is not suitable for the structure of the head chip 1a including the heating elements 12 aligned close to the first side surface of the semiconductor substrate 11.
The influences of development of bubbles on the first side surface of the head chip 1a will now be described. FIG. 21 is a cross-sectional view of the head chip 1a shown in FIG. 18 showing the state where bubbles are generated. FIG. 21 shows the head chip 1a when it is actually used and so the elements shown in FIG. 18 are inverted in FIG. 21. As described above, in the semiconductor substrate 11, bubbles are generated the most at a portion whose temperature is highest in the region where bubbles are generated (bubbling region) shown in FIG. 21. This portion is in contact with ink and bubble nuclei exist therein. This portion is the lowermost part in the bubbling region in FIG. 21.
Theoretically, bubbles generated in ink move upward by its buoyancy. In actual use, however, ejection of ink drops reduces the amount of ink in the reservoir 3a. Accordingly, ink in the bubbling region is drawn towards the nozzle 18, that is, towards the reservoir 3a, and the bubbles are also drawn towards the common flow path 23 and the individual flow path 3d. 
FIG. 22 is an enlarged photograph of the head 1 including the transparent nozzle sheet having the same structure as that of the nozzle sheet 17. The photograph in FIG. 22 is taken immediately after liquid drops are ejected and shows the generation of bubbles. White dots in FIG. 22 are bubbles, whereas black dots are spatters of ejected ink drops.
Even when the number of bubbles generated in the individual flow paths 3d and the common flow path 23 close to the individual flow paths 3d is very small, ejection of ink may be influenced by these bubbles to some extent. When the number of generated bubbles is large, small bubbles may be united into larger bubbles. In this case, the surface tension of the bubbles decreases the amount of ink supplied to narrow flow paths, that is, the individual flow paths 3d. Moreover, ink cannot flow into the individual flow paths 3d at all in some cases. FIG. 23 is an enlarged photograph of the head 1, showing the region where ink supply is decreased because some small bubbles are united into larger bubbles.
Due to a decrease in the amount of ink supplied to the individual flow path 3d, a sufficient amount of ink cannot be ejected as ink drops. Moreover, sometimes no ink is ejected from a nozzle at all. A serial head for a serial printer prints an image or character by multiple ink ejection by being slightly moved while printing and thus the amount of ejected ink can be evened out over the print sheet. Thus, failure in ink ejection is not noticeable. On the other hand, a line head for a line printer prints an image or character by a single ink ejection. Therefore, when the line head encounters failure in ink ejection, the resulting printing has a line (white line) at a position corresponding to the part of the head suffering from the failure.
FIG. 24 is an enlarged photograph of a line head, showing a white line formed due to lack of ink supply to the reservoirs 3a, which is caused by the generation of bubbles. In FIG. 24, ejection failure occurs in the width for about four nozzles out of the entire width of about 2.7 mm for 64 nozzles.